Sure, you can get a special geeky engagement ring for your special lady, whether you get it custom made or make it yourself, but if you don’t have the right box for it, your plan might fall flat. This TARDIS engagement ring box beats any ring box ever!
Come to think of it, this ring box is a good metaphor for the Doctor and his companions. He shows them a special box. They get excited. Off they go on their adventures. Eventually when he has tired of them or when they have had enough, it’s all over.(Sometimes they even lose their memory of the whole thing like Donna. Or they go off into a parallel dimension with a clone of the Doctor like Rose.) The Doctor wallows in self pity for a bit and finds another (often younger) model.
Yes, it’s kind of like the Doctor has been married a hundred times, but the dirty old codger keeps on trying.